1. Field of the Invention
The invention relates generally to a current-perpendicular-to-the-plane (CPP) magnetoresistive (MR) sensor that operates with the sense current directed perpendicularly to the planes of the layers making up the sensor stack, and more particularly to a scissoring-type CPP sensor with dual sensing or free layers.
2. Background of the Invention
One type of conventional MR sensor used as the read head in magnetic recording disk drives is a “spin-valve” sensor based on the giant magnetoresistance (GMR) effect. A GMR spin-valve sensor has a stack of layers that includes two ferromagnetic layers separated by a nonmagnetic electrically conductive spacer layer, which is typically copper (Cu) or silver (Ag). One ferromagnetic layer adjacent the spacer layer has its magnetization direction fixed, such as by being pinned by exchange coupling with an adjacent antiferromagnetic layer, and is referred to as the reference layer. The other ferromagnetic layer adjacent the spacer layer has its magnetization direction free to rotate in the presence of an external magnetic field and is referred to as the free layer. With a sense current applied to the sensor, the rotation of the free-layer magnetization relative to the reference-layer magnetization due to the presence of an external magnetic field is detectable as a change in electrical resistance. If the sense current is directed perpendicularly through the planes of the layers in the sensor stack, the sensor is referred to as a current-perpendicular-to-the-plane (CPP) sensor.
In addition to CPP-GMR read heads, another type of CPP-MR sensor is a magnetic tunnel junction sensor, also called a tunneling MR or TMR sensor, in which the nonmagnetic spacer layer is a very thin nonmagnetic tunnel barrier layer. In a CPP-TMR sensor the tunneling current perpendicularly through the layers depends on the relative orientation of the magnetizations in the two ferromagnetic layers. In a CPP-GMR read head the nonmagnetic spacer layer is formed of an electrically conductive material, typically a metal such as Cu or Ag. In a CPP-TMR read head the nonmagnetic spacer layer is formed of an electrically insulating material, such as TiO2, MgO, or Al2O3.
A type of CPP sensor has been proposed that does not have a ferromagnetic reference layer with a fixed or pinned magnetization direction, but instead has dual ferromagnetic sensing or free layers separated by a nonmagnetic spacer layer. In the absence of an applied magnetic field, the magnetization directions or vectors of the two free layers are oriented generally orthogonal to one another with parallel magnetization components in the sensing direction of the magnetic field to be detected and antiparallel components in the orthogonal direction. With a sense current applied perpendicularly to the layers in the sensor stack and in the presence of an applied magnetic field in the sensing direction, the two magnetization vectors rotate in opposite directions, changing their angle relative to one another, which is detectable as a change in electrical resistance. Because of this type of behavior of the magnetization directions of the two free layers, this type of CPP sensor will be referred to herein as a “scissoring-type” of CPP sensor. If a CPP-GMR scissoring-type sensor is desired the nonmagnetic spacer layer is an electrically conducting metal or metal alloy. If a CPP-TMR scissoring-type sensor is desired the spacer layer is an electrically insulating material. In a scissoring-type CPP sensor, a single layer of hard magnetic material at the back of the sensor, opposite the air-bearing surface, is used to bias the magnetization directions so that they are roughly orthogonal to one another in the quiescent state, i.e., in the absence of an applied magnetic field. Without the hard bias layer, the magnetization directions of the two free layers would tend to be oriented antiparallel to one another. This tendency to be oriented antiparallel results from strong magnetostatic interaction between the two free layers once they have been patterned to sensor dimensions, but may also be the result of exchange coupling between the magnetic layers through the spacer. The scissoring-type of CPP sensor is described in U.S. Pat. No. 7,035,062 B2. Unlike in a conventional CPP GMR or TMR sensor, in a scissoring-type CPP sensor there is no need for an antiferromagnetic pinning layer. Accordingly, the read-gap and parasitic series electrical resistances are greatly reduced. This enables an enhanced down-track resolution and a stronger magnetoresistance signal.
In a scissoring-type CPP sensor, the detected signal field is aligned collinearly with the bias field from the hard bias layer above the sensor, rather than orthogonally as in the case of a conventional GMR spin-valve type sensor with two hard bias layers on each side. In situations where the signal field is antiparallel to the bias field, the total applied field on the scissoring-type sensor is reduced in magnitude, and it is more susceptible to magnetic instability (particular that originating at track edges) than a spin-valve type sensor where the total applied field on the sensor is never smaller than the hard bias field (which is strongest at the track edges). This generally makes the stabilization of the scissoring-type sensor more difficult compared to a spin-valve sensor.
One technique that addresses this stabilization problem is directional ion milling of either the free layers or the layers on which the free layers are subsequently deposited. This results in additional uniaxial anisotropy in the free layers. This technique is described in U.S. Pat. No. 8,015,694 B2 which is assigned to the same assignee as this application. However, with this technique it can be difficult to precisely define the anisotropy axes and to achieve uniformity over an entire wafer, from which a large number of sensors are fabricated.
What is needed is a scissoring-type CPP sensor with improved stability, where the magnetization directions of the two free layers are more easily maintained generally orthogonal to one another in the quiescent state as a result of uniaxial anisotropy induced other than by etching.